The present invention relates to a process for producing a one-component alkoxy functional room temperature vulcanizable (room temperature vulcanizable shall hereinafter be referred to as RTV) silicone rubber composition, and more particularly, the present invention relates to a process for producing a one component alkoxy functional RTV composition utilizing a devolatilizing extruder.
One component alkoxy functional RTV silicone rubber compositions are well known as exemplified by Beers, U.S. Pat. No. 4,100,129. A major innovation in the processing of such composition was a process to anhydrously mix the ingredients prior to packaging in a twin-screw devolatilizing extruder such as disclosed in Beers, et al. U.S. Pat. No. 3,960,802. As pointed out in the foregoing patent, the mixing was carried out much more efficiently in said devolatilizing extruder. In addition, it was found that it was possible to obtain lower viscosities of the mixed composition by the use of a devolatilizing extruder. This was a major innovation in the production of alkoxy functional one component RTV compositions since it permitted the production of such compositions continuously at a somewhat lower cost than was possible with the prior mixing procedures.
Recently, there has been developed a one-component shelf stable alkoxy functional RTV silicone rubber composition such as for instance disclosed in White, et al. U.S. Pat. No. 4,395,526. This patent discloses the initial preparation of a poly-alkoxy terminated diorganopolysiloxane polymer. Once this polymer is prepared, there is added a scavenger to tie-up free alcohol and hydroxy groups in the uncured polymer mixture along with the addition of other ingredients, such as filler, adhesion promoters, condensation catalysts, etc. The innovation in this patent application is the disclosure that to the poly-alkoxy end-capped diorganopolysiloxane polymer there is added a scavenger subsequent to the formation of such a polymer so as to tie-up all hydroxy groups which would degrade it.
The production of such an uncured composition, in which the hydroxyl groups and alcohol in the mixture are tied up, results in a shelf stable composition; that is, even though the compositions of Beers, U.S. Pat. No. 4,100,129 and U.S. Pat. No. 3,960,802 were shelf stable, they still do not have a sufficiently fast cure rate after being stored at periods of time of six months or more. As a matter of fact, as the White et al. case points out, such compositions would suffer a degradation in shelf stability after being stored for periods of time of as little as two weeks.
Accordingly, as pointed out in the White, et al., U.S. Pat. No. 4,395,526 case, the shelf stability of the composition could be prolonged, and the alkoxy functional RTV composition which traditionally has a slow cure rate would have a much faster cure rate if the polyalkoxy terminated diorganopolysiloxane polymer was first formed and there was utilized in the composition a scavenger for hydroxy groups.
There were other developments in this area. For instance, note the patent application of Chung, Ser. No. 338,518, filed on Nov. 11, 1981, now U.S. Pat. No. 4,424,157, in which is disclosed the utilization of certain cyclic amides as scavengers. Further note the disclosure of Beers, Ser. No. 349,537, filed on Feb. 17, 1982, now abandoned which discloses the production of a fast curing shelf stable alkoxy functional composition by the utilization of certain scavengers, and particularly the scavengers of the White, et al. case, U.S. Pat. No. 4,395,526 and in addition, certain other additives which made the composition low modulus. Further, there is a disclosure of Dziark, U.S. Pat. No. 4,417,042, which patent application discloses the utilization of certain silazane scavengers for such compositions. Then, there is the disclosure of Lucas, et al., Ser. No. 349,538, filed on Feb. 17, 1982, now U.S. Pat. No. 4,483,973, which discloses the use of certain adhesion promoters for the alkoxy functional one component RTV systems of White, et al., U.S. Pat. No. 4,395,526 and for the Dziark patent, U.S. Pat. No. 4,417,042 and Beers case, Ser. No. 349,537. There have been further additional developments in the field. For instance, note the disclosure of Chung, et al. entitled, "NOVEL SCAVENGERS FOR ONE COMPONENT RTV COMPOSITIONS", Ser. No. 428,038, filed Sept. 29, 1982, which discloses the use of certain alkoxy functional silazanes as either scavengers or integrated scavengers and cross-linking agents. In addition, there is the disclosure of Chung, entitled, "END-CAPPING CATALYSTS FOR FORMING ALKOXY-FUNCTIONAL ONE COMPONENT RTV COMPOSITIONS", Ser. No. 427,930, filed Sept. 29, 1982, now abandoned, which discloses the utilization of certain acidic catalysts and a combination of acidic and amine catalysts as coupling catalysts for the end-capping of the polyalkoxy terminated diorganopolysiloxane polymer. All these compositions were highly desirable innovations in the production of shelf stable fast curing low modulus one component RTV silicone rubber compositions.
One point should be made clear. Most of the compositions of Beers, U.S. Pat. No. 4,100,129 and Beers, et al., U.S. Pat. No. 3,960,802 would cure after a period of time of a year or even as long as two years. However, the cure rate could be substantially affected and after long periods of storage would be prolonged to an undesirable amount so that the composition was not always simple, efficient, or economical to work with. An improvement of the foregoing White, et al case, U.S. Pat. No. 4,395,526 as well as the other cases, was to produce a shelf stable composition, that is, a composition with a cure rate which did not degrade as much after the composition was stored as did the cure rate of the composition of the foregoing Beers, U.S. Pat. No. 4,100,129, and that of Beers, et al., U.S. Pat. No. 3,960,802. Accordingly, you have a more shelf stable, fast curing composition, with the compositions of the foregoing White, et al., U.S. Pat. No. 4,395,526 and Dziark, et al., U.S. Pat. No. 4,417,042, patents as well as the other cases dealing with the new innovations in the alkoxy functional RTV compositions than you did with the older compositions of the foregoing Beers patents.
It was highly desirable in the method of making such compositions that the silanol end-stopped base polymer be mixed with the cross-linking agent and the end-coupling catalysts without a scavenger in a first mixing step and then in a second mixing step, the condensation catalyst mixture was mixed along with the scavengers.
A much more efficient mixing procedure for such compositions was desirable and particularly a continuous mixing procedure. If a continuous mixing procedure was developed for the mixing of such compositions, then there would be a resultant savings in labor as well as the production of a larger amount of material in a given time.
It is one object of the present invention to provide a continuous mixing procedure for the production of alkoxy functional one component RTV compositions having scavengers in them.
It is another object of the present invention to provide an efficient process for producing a shelf stable, fast curing, alkoxy functional RTV silicone rubber composition in which the mixing is carried out by a devolatilizing extruder.
It is still a further object of the present invention to provide an efficient, continuous mixing process which results in a low viscosity fast curing shelf stable alkoxy functional one component RTV silicone rubber composition, by first mixing and forming the end-capped polyalkoxy diorganopolysiloxane in a static mixer and taking such a polymer and mixing the other ingredients such as filler, in a devolatilizing extruder.
These and other objects of the present invention are accomplished by means of the disclosure set forth herein below.